build(deps): bump gunicorn from 20.1.0 to 21.2.0 in /flask

Bumps [gunicorn](https://github.com/benoitc/gunicorn) from 20.1.0 to 21.2.0.
- [Release notes](https://github.com/benoitc/gunicorn/releases)
- [Commits](benoitc/gunicorn@20.1.0...21.2.0)

---
updated-dependencies:
- dependency-name: gunicorn
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

flask/requirements.txt

@@ -9,7 +9,7 @@ Flask-SocketIO==5.3.4
 greenlet==2.0.2
 # Still no point release containing PR #2581
 # see https://github.com/benoitc/gunicorn/pull/2581
-https://github.com/benoitc/gunicorn/archive/ff58e0c6da83d5520916bc4cc109a529258d76e1.zip#egg=gunicorn==20.1.0
+https://github.com/benoitc/gunicorn/archive/ff58e0c6da83d5520916bc4cc109a529258d76e1.zip#egg=gunicorn==21.2.0
 h11==0.14.0
 itsdangerous==2.1.2
 Jinja2==3.1.2